Lesson 13: Security

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se·cu·ri·ty ( siˈkyo͝oritē/ ) [ noun ]

The state of being free from danger or threat.

The safety of a state or organization against criminal activity such as terrorism, theft, or espionage.

Computer Security is a big topic. DOBC will only be able to scratch the surface, but if you’re interested at all you should 100% do more research on the topic. It is a fascinating field and well worth knowing about, even if you’re not intending to be a security professional.

Types of Security

XKCD on WiFi Security

There are three main types of security in computing:

Physical Security
Use physical barriers to prevent unauthorized access to data
Software Security
Fix flaws in your application that could grant attackers unwanted levels of access to your systems
Network Security

Security pertaining to networked services (websites, databases, etc).

  • Active: in which an intruder initiates commands to disrupt the network’s normal operation (Denial-of-Service, Ping of Death)
  • Passive: a network intruder intercepts data traveling through the network. (Man-in-the-Middle, Wiretapping, Idle Scan)

Each of these encompasses a field of computer security unto itself. We will at least mention each of them in more detail, but we will focus on network security in this course.

Threat Models

Threat models allow you to focus and limit your security resources on what is necessary instead of what is possible.

Threat models are the assessment of which attacker you are protecting against. This is so you don’t spend too much time in a panic attack trying to protect your tiny webapp from the NSA.

For example, your threat model might be protecting against a DDOS attacker. You’re not worrying about the multitude of other types of attacks there are out there, you’re just focusing on the person that’s going to bombard your server with too many requests. There are many ways to protect against DDOS attacks that a developer could reasonably implement, allowing one to take action against a threat instead of just losing sleep over it.

Access Control

XKCD Identity Comic
  • Identification: Who is this person?
  • Authentication: Is this person who they say they are?
  • Authorization: Is this person allowed to perform this action?

Access Control is a framework for controlling who has access to what resources on a system. There are many ways to implement Access Control, but the three basic principles of Access Control are Identification, Authentication, and Authorization.


Who is this person?

Identification is the first step in granting access. During this step, the user identifies themselves to the system they wish to access. One example of an identifying piece of information is a username. In most cases, however, identification isn’t enough. It’s easy enough to claim to be someone that you aren’t, which is why you have to perform Authentication alongside identification.


Is this person who they say they are?

An example of Authentication would be asking for a password or passphrase. When you are logging into a website or computer you are authenticating.


Is this person allowed to perform this action?

An example of Authorization is when you try to open a file on a shared computer and you are denied access. Your user (that you authenticated as) is not allowed to access that file.

Passwords / Passphrases

Passwords are the de facto form of Authentication for computers, but they aren’t a perfect solution (to an admittedly difficult problem).

Passwords are hard to remember and surprisingly easy for computers to guess (because people are bad at making up creative ones). That’s okay though, the problem of passwords has been studied and there are some solutions that anybody can use.

Problems with Passwords

Passwords are a necessary part of security. They aren’t great though for a few reasons.

  • People repeat passwords.
  • Many passwords are easy to guess.
  • Passwords are hard to remember.

Solutions for Passwords

Thankfully because passwords have been a problem for so long there are a few workarounds including:

Use a password manager.
  • LastPass is very popular and convenient.
  • pass on Linux and OSX, uses FOSS tools to store passwords.
Change your passwords regularly.
Warning this is considered a ‘Security Best Practice’, but only if you create a unique password each time you change it. Don’t replace your password by adding a 1 to the end of it.
Use multi-step authentication.
Google, Github, and Steam (as well as many other services) offer multi-step authentication. This is usually implemented with a secondary password which is generated every thirty seconds that you enter after your actual password. The second password changes so often, and is only visible on your devices, so it makes sneaking into your account much more difficult.

Choosing Pass-phrases

Let the password manager generate them for you.
You might like to create your own passwords, but you are far worse than a computer at generating them. Anything you can remember easily is convenient for a computer to guess, and anything that isn’t easy to remember is just a waste of your time.
Use pass phrases instead of words.

When you can’t use a password manager consider making your password a sentence or phrase instead of a word. This is harder for a computer to guess and easy for you to remember.

For instance the dog jumped over 85 cats is more secure and easier to remember than xc456zkqt55, and way more secure.

Relevant funny bash.org post

XKCD passwords comic

Certificates and HTTPS

HTTPS Lock in Browser URL Bar

Certificates are what allows your computer to create a secure connection with a server and transfer sensitive data across the wire.


Hyper Text Transfer Protocol Secure.

This is an extension of the HTTP protocol designed for secure web communication, but it’s a good idea to ensure that you’re using it everywhere by replacing http:// with https:// or by using a browser extension like HTTPS Everywhere.

Certificate Authorities
An entity that issues digital certificates for HTTPS connections. These certificates are trusted by Browsers through a web of trust. Anybody can create their own certificates, but only certificate authorities can issue a certificate signed by a trusted organization.

Secure Socket Layer/Transport Layer Security.

This is the actual protocol used for secure connections over the web. TLS is the new protocol that replaced SSL, but they are sometimes discussed as one / interchangeably.

To learn more about a website’s HTTPS certificate and security info, click the little Lock icon in the URL bar. You should be able to find more information about the certificate including which authority it comes from, and more information about it.

Types of Attacks

Frequency of online attacks (37% Cross Site Scripting, 16% SQL Injection, etc)

Code Injection

Billy Droptables XKCD Comic

Code Injection is the act of inserting code into a running process (website, webapp, word processor, etc.) with malicious intention.

Code Injection Attacks

SQL Injection:
SQL Injection is when you take advantage of the fact that a form input is inserted directly into a SQL query. You write some password and then write a new SQL query which drops all tables, or returns all data, exploiting an easy security hole.
| username: | admin                                  |
| password: | pass' || true); DROP TABLE STUDENTS;-- |
Cross-Site Scripting (XSS):
Cross-Site Scripting is when a malicious script is sent to, and run on, a person’s computer. This tends to take advantage of the fact that your browser blindly runs any JavaScript you tell it to.
<img onerror=alert("Tracking your IP with a GUI interface!");>
Cross-Site Request Forgery (CSRF):
CSRF is when one website on your browser tries to carry out an action as you on a different website. For instance you’re an admin of some big social media website, you get an email, embedded in the email is a CSRF script which tries to delete all user accounts on your website. Since you’ve got your credentials cached your browser doesn’t know better and can run that command because it looks like any other command.
<img src="http://example.com/?action="Delete All Accounts">

Code Injection Defenses

  • Sanitize User Inputs
  • Use CSRF Tokens

Some of these attacks are very hard to fight against, but they all have industry-tested solutions that are easy enough to implement in an application of your own.

Sanitize Inputs
Input sanitation is when your code sniffs a piece of input to see if it looks like a SQL or code of any kind. If it does look like code it’s probably malicious so your program errors out and tells the user to enter a real input.
CSRF Tokens
A CSRF token is a unique string that has to be tied to each request you send to a server. You don’t need to log back in each time you get a new one but the application won’t complete your action unless the token is included in your query. This means only the website you’re logged into can send a real query because only that website knows the CSRF token.

Web Server Attacks

Apache Version Vulnerability

Web Server attacks take advantage in vulnerabilities of specific versions or default configurations of webservers.

For instance a webserver’s default configuration might allow you to turn it off by sending a special request. The config file tells you to change that special request but most people don’t.

As far as versions go, software has bugs. Developers do their best to fix those bugs but when you’re running a big website and your infrastructure is fragile you don’t usually want to update anything including the webserver software you’re running. So despite the bug being fixed, not everybody is on the same page.

Discovering Vulnerabilities

As you begin to grow and push the technology you’re using you might find real vulnerabilities that could affect real people. In that case, take a deep breath and take responsible action. Here is a good set of guidelines to follow if that happens:

  1. Test and document the bug to verify it exists.
    If you think you encountered a bug, make sure you can replicate it. If you can’t how can you expect the developers to recreate it?
  2. Disclose it privately to those responsible for fixing it.
    Provide examples – it’s basically a bug report, but through private channels (not public tracker yet!)
  3. Give them time to release a patch before announcing it.
    Google waits 90 days to announce a bug after informing the developers.

If you are interested in making a pretty penny off of your bug, many projects have bug bounties which give a $$$ reward for bugs.

Further Reading

Try your hand at actual SQL Injection attacks
OverTheWire Wargames
Learn the basics of offensive security by solving challenges and using exploits to gain access to the password for the next level.